Pretty Good Privacy

Pretty Good Privacy (PGP), or OpenPGP, makes it possible for people to encrypt their email and other communications over the internet.

PGP offers end-to-end encryption, so its trust model does not require that users trust the service provider. It is sufficient that the user trusts his or her computer, and the computer of the receiver. This is unlike server-side encryption solutions, such as that used by Outlook.com (the NSA reads your encrypted Outlook messages).

Furthermore, PGP implementations such as GnuPG and APG are trustworthy because they are open source software. In contrast, closed source software may claim to offer secure encryption, but that is very difficult to prove. Fortunately, in some cases these security claims are easy to disprove: all Skype messages are tapped by Microsoft.

Contents

Motivation

The internet without encryption is synonymous with a crowd of people yelling at each other; a curious eavesdropper can easily walk up and listen. It would simply be too dangerous to transmit obviously sensitive information over the internet, such as social security numbers or online banking passwords. You may try to encrypt those highly sensitive communications, but dedicated eavesdroppers can collect the rest of your unencrypted internet communications to paint a detailed picture of your personality and habits. For more on motivations, see our Internet Surveillance page.

Using GNU Privacy Guard

Note: This section is a work in progress.

This section is geared towards beginners who have never used PGP software. It will cover setup, and an example conversation.

Setup

create your PGP key pair

share your public key with your friend

obtain the public key of your friend and import it into your keyring

Example conversation

compose message to your friend

click the "PGP Encrypt" check box

send message

provide the passphrase for your PGP private key

Theory

The following is a high level description of the nuts and bolts of PGP, and is not necessary to understand if you just want to use it.

Two typical methods of encrypting data include 1) secret-key and 2) public-key. PGP uses public-key cryptography, but both methods are introduced for comparison.

Secret-key (symmetric) cryptography—Alice wants to send Bob a secret message. They both agree on a single secret password to use as an encryption/decryption key. Alice can use that key to encrypt data to send to Bob, and Bob can use that key to decrypt data received from Alice. Mathematically, a symmetric cipher is just a one-to-one function that is easily invertible—apply the function to encrypt data, or apply its inverse decrypt data. The key describes the characteristics of the function; without the key, one cannot determine the function or its inverse.

Symmetric cryptography has some disadvantages:

At the very beginning, when Alice and Bob initially exchanged the key, the exchange could have been transparently intercepted by a third party (assume Alice and Bob were not using a secure channel to exchange the key because that is precisely what we are trying to build!). With a stolen key, a third party can eavesdrop on Alice or Bob, and even pretend to be Alice or Bob.

It is unwieldy for large groups of people to communicate privately, since the number of keys is polynomial with the number of people (recall the number of edges in a complete graph of n nodes):

# people

# keys

2

1

3

3

4

6

5

10

6

15

n

n(n-1)/2

A series of complete graphs from n=2 to 15. Each edge represents a secret key that has been exchanged between two people (nodes).

Public-key (asymmetric) cryptography—rather than just using one key, a public-key cipher uses a pair of keys for sending messages. One is a public key used for encryption, and the other is a private key used for decryption. Anybody, including Alice, can use Bob's public key to encrypt messages, but those messages can only be decrypted with the corresponding private key which Bob keeps to himself. If Alice also has a public/private key pair then she can send and receive secret messages to and from Bob.

Only the public (decryption) key needs to be sent over the network, so an eavesdropper cannot simply steal the key by performing a man-in-the-middle attack.

Large groups of people can easily communicate because each person only needs to generate his or her own key pair. The number of key pairs in the network is linear with the number of people (2n). Alice never needed to negotiate with Bob to agree on a shared secret key—they each simply generated their own key pairs and grabbed each other's public key.

Warnings

As stated in the introduction, PGP's trust model requires that the user trusts his or her computer, which means trusting all the software on his or her computer. The use of open source software on a computer is a prerequisite for trusting the computer, so we advise against the use of untrustworthy proprietary systems such as Microsoft Windows or Mac OS X. Instead, we recommend GNU/Linux or BSD. If you are not already using an open source operating system, see our Getting started page for some guidance.